Concentration-insensitive and low-driving-voltage OLEDs with high efficiency and little efficiency roll-off using a bipolar phosphorescent emitter

A series of simplified trilayer phosphorescent organic light-emitting diodes (PHOLEDs) with high efficiency and little efficiency roll-off based on a bipolar iridium emitter Iridium(III) bis(2-phenylpyridinato)-N,N′-diisopropyl-diisopropyl-guanidinate (ppy)₂Ir(dipig) has been demonstrated. They are dominated by the efficient direct-exciton-formation mechanism and show gratifying concentration-insensitive and low-driving-voltage features. In particular, very high and stable electroluminescence (EL) efficiencies (maximum power efficiency and external quantum efficiency >98 lm W^?1 and 25% respectively, and external quantum efficiency >20% over a wide luminance range of 1–15,000 cd m^?2) are achieved in the PHOLEDs based on emitting layers (EMLs) consisting of (ppy)₂Ir(dipig) codeposited with common host CBP in an easily controlled doping concentration range (15–30 wt%). The EL performance of the PHOLEDs is comparable to the highest PHOLEDs reported in scientific literature.     

High-efficiency and good color quality white light-emitting devices based on polymer blend

Here we report efficient and color-stable white polymer light-emitting devices (WPLEDs) based on a newly synthesized efficient blue emitting polymer poly[(9,9-bis(4-(2-ethylhexyloxy)phenyl)fluorene)-co-(3,7-dibenziothiene-S,S-dioxide10)] (PPF-3,7SO10) which dually function as host material and blue emitter, with appropriate blending ratio with two typical electroluminescent polymers, green emitting poly[2-(4-(3′,7′-dimethyloctyloxy)-phenyl)-p-phenylenevinylene] (P-PPV) and orange–red emitter poly[2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH–PPV) with appropriate blending ratio. In a single active layer WPLEDs with a blending ratio of 100:0.8:0.5 (B:G:R) by weight, white light emission with CIE coordinate of (0.34, 0.35) was realized. The resulted device shows a high luminous efficiency (LE) of 8.7 cd A^?1, which could be further enhanced to 14.0 cd A^?1 with incorporation of a thin hole transporting layer poly (vinylcarbazole) (PVK) at the anode side. The obtained luminous efficiency is listed as one of the highest reported value for WPLEDs based on all fluorescent polymer emitters. The devices had appropriate color temperature of 2500–6500 K and high color rendering index (CRI) of 72–79, and are characterized with stable electroluminescent spectra upon change of current density, stress and annealing at high temperature, thus can find application in solid-state lighting.     

High-efficiency hybrid organic–inorganic light-emitting devices

We report efficient red, orange, green and blue organic–inorganic light emitting devices using light emitting polymers and polyethylenimine ethoxylated (PEIE) interlayer with the respective luminance efficiency of 1.3, 2.7, 10 and 4.1 cd A⁻¹, which is comparable to that of the analogous conventional devices using a low work-function metal cathode. This is enabled by the enhanced electron injection due to the effective reduction of the ZnO work-function by PEIE, as well as hole/exciton-blocking function of PEIE layer. Due to the benign compatibility between PEIE and the neighboring organic layer, the novel phosphorescent organic–inorganic devices using solution-processed small molecule emissive layer show the maximum luminance efficiency of 87.6 cd A⁻¹ and external quantum efficiency of 20.9% at 1000 cd m⁻².     

Simple-structured efficient white organic light emitting diode via solution process

High-efficiency white emission is crucial to the design of energy-saving display and lighting panels, whereas solution-process feasibility is highly desirable for large area-size and cost-effective roll-to-roll manufacturing. In this study, we demonstrate highly-efficient, bright and chromaticity stable white organic light emitting diodes (OLEDs) with solution-processed single emissive layer. The resultant best white OLED shows excellent electroluminescence performance with forward-viewing external quantum efficiency, current efficiency and power efficiency of 22.7%, 48.8 cd A^− 1 and 27.8 lm W^− 1 at 100 cd m^− 2, respectively, with a maximum luminance of 19,590 cd m^− 2. Furthermore, we also observed an increment of 112% in the power efficiency, 86.9% in the current efficiency and a decrement of 39.2% in the external quantum efficiency at 100 cd m^− 2 as the doping concentration of blue dye was increased from 10 wt% to 25 wt% in the devices. The better efficiency performance may be attributed to the effective exciton-confining device architecture and low-energy barrier for electrons to inject from the hole-blocking electron-transport layer to the host layer.     

Enabling high-efficiency organic light-emitting diodes with a cross-linkable electron confining hole transporting material

Wet-process enables flexible, large area-size organic devices to be fabricated cost-effectively via roll-to-roll manufacturing. However, wet-processed devices often show comparatively poor performance due to the lack of solution-process feasible functional materials that exhibit robust mechanical properties. We demonstrate here a cross-linkable material, 3,6-bis(4-vinylphenyl)-9-ethylcarbazole (VPEC), to facilitate the injection of hole and meanwhile effectively confine electron to realize, for examples, high efficiency organic light-emitting diodes, especially at high luminance. The VPEC shows a hole mobility of 1 × 10⁻⁴ cm² V⁻¹ s⁻¹ and a triplet energy of 2.88 eV. Most importantly, the VPEC not only works for devices containing low band-gap red or green emitters, but also for the counterpart with high band-gap blue emitter. With the electron confining hole transporting material, the power efficiency of a studied red device, at 1,000 cd m⁻² for example, is increased from 8.5 to 13.5 lm W⁻¹, an increment of 59%, and the maximum luminance enhanced from 13,000 to 19,000 cd m⁻², an increment of 46%. For a high triplet energy blue emitter containing device, it is increased from 6.9 to 8.9 lm W⁻¹, an increment of 29%, and the maximum luminance enhanced from 9,000 to 11,000 cd m⁻², an increment of 22%.     

Indolo[3,2-b]carbazole: Promising building block for highly efficient electroluminescent materials

Two luminescent materials based on indolo[3,2-b]carbazole have been designed and synthesized. They were highly fluorescent both in solution and in the solid state. High-performance electroluminescent devices with indolo[3,2-b]carbazole luminescent derivatives as the emissive materials were fabricated for the first time with low turn-on voltage of 2.65 V, high luminescence efficiency of 7.92 lm W^?1, and high brightness of 68729 cd m^?2. The results demonstrated that indolo[3,2-b]carbazole has great potentials as promising building block for highly efficient electroluminescent materials.     

Novel dibenzothiophene based host materials incorporating spirobifluorene for high-efficiency white phosphorescent organic light-emitting diodes

Two host materials, DBTSF2 and DBTSF4, were designed and synthesized, incorporating dibenzothiophene (DBT) and spirobifluorene (SF) blocks. Their thermal, electrochemical and photo-physical properties were fully characterized. DBTSF4, which adopted an ortho-linkage between DBT and SF moieties, showed a significantly higher T₁ energy of 2.82 eV as compared to its para-linkage analogue DBTSF2 (2.49 eV). Their applications as host for green, blue and white phosphorescent organic light-emitting diodes (PHOLEDs) were explored. The DBTSF4 based blue PHOLED has a highest current efficiency of 23.5 cd A^?1. And using DBTSF4 as a single host, two-color based white PHOLEDs were achieved from cold white emission with CIE coordinate of (0.31, 0.43) to yellowish warm white emission (0.44, 0.49) with maximum current efficiencies varying from 35.8 to 52.3 cd A^?1 and maximum external quantum efficiencies from 13.1% to 16.9% respectively. The white PHOLED devices also showed a low efficiency roll-off even at 10,000 cd m^?2.     

Efficient solution-processed electrophosphorescent devices using ionic iridium complexes as the dopants

Efficient solution-processed electrophosphorescent devices using two blue-emitting ionic iridium complexes (complex 1 and complex 2) were fabricated, with poly(N-vinylcarbazole) (PVK):1,3-bis(5-(4-tert-butylphenyl)-1,3,4-oxadiazol-2-yl)benzene (OXD-7) as the host and Cs₂CO₃/Al as the cathode. Using complex 1 as the dopant, we obtained efficient blue-green electrophosphorescence from single-layer devices with a maximum efficiency of 12.2 cd A^?1, a maximum brightness of 12,600 cd m^?2 and CIE (Commission Internationale de l’Éclairage) coordinates of (0.19, 0.45). And the maximum efficiency of the device based on complex 1 can be further improved to 20.2 cd A^?1, when a thin 1,3,5-tris(1-phenyl-1H-benzo[d]imidazol-2-yl)benzene (TPBI) layer was inserted between the light-emitting layer and the cathode. Using complex 2 as the dopant, we obtained deep-blue electrophosphorescence with the emission peak at 458 nm and CIE coordinates of (0.16, 0.22). Our work suggests that ionic iridium complexes are promising phosphors for obtaining efficient electrophosphorescence in the blue region.     

Highly efficient and color stable single-emitting-layer fluorescent WOLEDs with delayed fluorescent host

We demonstrated highly efficient and color stable single-emitting-layer fluorescent WOLEDs using blue thermally activated delayed fluorescent material of bis[4-(9,9-dimethyl-9,10-dihydroacridine)phenyl]sulfone (DMAC-DPS) as host and traditional orange fluorescent material of (5,6,11,12)-tetraphenyl-naphthacene (rubrene) as dopant. At a low dopant concentration of 0.6 wt%, we achieved the efficient white emission that comprised of blue host and orange dopant. The maximum current efficiency, power efficiency and external quantum efficiency were 20.2 cd A⁻¹, 15.9 lm W⁻¹ and 7.48%, respectively. Besides, the Commission Internationale de I’Eclairage coordinates were almost the same with the increased voltage, which shifted from (0.359, 0.439) to (0.358, 0.430) as the voltage rose from 5 V to 8 V. The achievement of so high efficiency was attributed to the efficient up-conversion of DMAC-DPS triplet excitons and efficient energy transfer from host to dopant by Förster transfer mechanism. The more detailed working mechanism was also argued.     

A highly efficient OLED based on terbium complexes

A neutral ligand 9-(4-tert-butylphenyl)-3,6-bis(diphenylphosphineoxide)-carbazole (DPPOC) and its complex Tb(PMIP)₃DPPOC (A, where PMIP stands for 1-phenyl-3-methyl-4-isobutyryl-5-pyrazolone) were synthesized. DPPOC has a suitable lowest triplet energy level (24,691 cm^?1) for the sensitization of Tb(III) (⁵D₄: 20,400 cm^?1) and a significantly higher thermal stability (glass transition temperature 137 °C) compared with the familiar ligand triphenylphosphine oxide (TPPO). Experiments revealed that the emission layer of the Tb(PMIP)₃DPPOC film could be prepared by vacuum co-deposition of the complex Tb(PMIP)₃(H₂O)₂ (B) and DPPOC (molar ratio = 1:1). The electroluminescent (EL) device ITO/N,N′-diphenyl-N,N′-bis(1-naphthyl)-1,1′-diphenyl-4,4′-diamine (NPB; 10 nm)/Tb(PMIP)₃ (20 nm)/co-deposited Tb(PMIP)₃DPPOC (30 nm)/2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP; 10 nm)/tris(8-hydroxyquinoline) (AlQ; 20 nm)/Mg_0.9Ag_0.1 (200 nm)/Ag (80 nm) exhibited pure emission from terbium ions, even at the highest current density. The highest efficiency obtained was 16.1 lm W^?1, 36.0 cd A^?1 at 6 V. At a practical brightness of 119 cd m^?2 (11 V) the efficiency remained above 4.5 lm W^?1, 15.7 cd A^?1. These values are a significant improvement over the previously reported Tb(PMIP)₃(TPPO)₂ (C).     

Highly efficient green phosphorescent organic light emitting diodes with simple structure

A series of simple structures is investigated for realization of the highly efficient green phosphorescent organic light emitting diodes with relatively low voltage operation. All the devices were fabricated with mixed host system by using 1,1-bis[(di-4-tolylamino)phenyl]cyclohexane (TAPC) and 1,3,5-tri(p-pyrid-3-yl-phenyl)benzene (TpPyPB) which were known to be hole and electron type host materials due to their great hole and electron mobilities [μ_h(TAPC): 1 × 10^?2 cm²/V s and μ_e(TpPyPB): 7.9 × 10^?3 cm²/V s] [1]. The optimized device with thin TAPC (5–10 nm) as an anode buffer layer showed relatively high current and power efficiency with low roll-off characteristic up to 10,000 cd/m². The performances of the devices; with buffer layer were compared to those of simple devices with single layer and three layers. Very interestingly, the double layer device with TAPC buffer layer showed better current and power efficiency behavior compared to that of three layer device with both hole and electron buffer layers (TAPC, TpPyPB, respectively).     

Energy transfer in polyfluorene copolymer used for white-light organic light emitting device

Luminescence properties of a type of polyfluorene copolymer (PFO–DBT5) used for white-light organic light emitting device (OLED) were studied and discussed, in which a very low concentration of 0.05 mol% 4,7-bithienyl-2,1,3-benzothiadiazole (DBT) molecules were inserted in polyfluorene chain as orange-light unit. From the spectroscopic analysis of PFO–DBT5 in solution and nanoparticles, it is concluded that inter-chain Förster energy transfer from polyfluorene segments to DBT units play major role in process of white-light emission of PFO–DBT5. By measurement of electroluminescent properties, it is found that annealing of PFO–DBT5 film in vacuum is favorable for improving white-light emission owing to enhancement in inter-chain Förster energy transfer. CIE coordinates (0.38, 0.33) were obtained under the annealing temperature of 140 °C. It was found that the annealing temperature affects the luminance and efficiency of white-light OLED. The optimum annealing temperature was 100 °C.     

Organic semiconductor heterojunction as charge generation layer in tandem organic light-emitting diodes for high power efficiency

High-performance tandem organic light-emitting diodes (OLEDs) employing a buffer-modified C₆₀/pentacene organic semiconductor heterojunction (OHJ) as a charge generation layer (CGL) are demonstrated. The unique cooperation of charge generation, transport, and extraction processes occurred in the OHJ-based CGL remarkably reduces the operational voltage. As a result, an approximately twofold enhancement in power efficiency (21.9 lm W^?1 VS 10.1 lm W^?1) can be achieved that has previously been suggested to be difficult for tandem OLEDs. When the pentacene is replaced by zinc phthalocyanine (ZnPc), copper phthalocyanine (CuPc), or phthalocyanine (H₂Pc), a similar power efficiency improvement can be also achieved. The novel design concept of the buffer-modified OHJ-based CGL is superior to that of the conventional CGLs. The investigations on the operational mechanism are performed, from which it is found that the mobile charge carriers firstly are needed to be accumulated at both sides of the heterojunction interface and then transport along the two organic semiconductors in terms of their good carrier transport characteristics under an external electrical field, and finally inject into the corresponding electroluminescent (EL) units by the interfacial layers.     

Morphology-dependent electroluminescence in poly(N-vinyl carbazole)-based multi-component single emissive layer polymer light-emitting diodes

We investigate the electroluminescent performance of single-layer polymer light-emitting diodes with the emissive layer comprised of poly(N-vinyl carbazole) (PVK): electron-transporting molecules (ETMs):iridium(III) [bis(4,6-difluorophenyl)-pyridinato-N,C²]-picolinate (FIrpic) trinary components. A series of ETMs with comparable energy levels is tentatively utilized to modulate the morphology. The morphology of the emissive layer is studied using the transmission electron microscopy and atomic force microscopy in combination with the interfacial energy calculation between these components, with purpose to understand the multi-component miscibility and morphological properties of these emissive layers. It is found that the multi-component miscibility has dramatic influence on the morphology of the multi-component emissive layer and the final electroluminescent performance of the devices. The results reveal that the aggregation of ETMs mainly results in an increase of driving voltages, while the aggregation of FIrpic emitters would lead to the reduction in the luminous efficiencies as a result of their self-quenching effects. The PVK:1,3-bis[(p-tert-butyl)phenyl-1,3,4-oxadiazoyl]benzene (OXD-7): FIrpic blend demonstrates a more uniform morphology resulting in an optimal luminous efficiency of 15 cd A^?1.     

Tuning the energy gap and charge balance property of bipolar host by molecular modification: Efficient blue electrophosphorescence devices based on solution-process

Three bipolar hosts composed of electron-accepting diphenylphosphine oxide and electron-donating carbazole/triphenylamine have been synthesized and characterized. With structural topology modification, the particular physical properties of the materials can be subtly optimized, such as the thermal stability, singlet–triplet energy gap and charge balance ability. Both DFT calculation and experiment results demonstrate that the introduced triphenylamine can effective minimize the HOMO–LUMO energy gap, while the carbazole units can prevent the excited energy loss and keep high triplet energy (E_T = 3.0 eV) due to the enhanced molecular rigidity. As a result, solution-processed blue PHOLEDs exhibited a high current efficiency of 25.2 cd A⁻¹ and a power efficiency of 11.5 lm W⁻¹, which implies that the unique molecular modulation is very cost-effective and competitive for the device performance improving.     

Alcohol-soluble polyfluorenes containing dibenzothiophene-S,S-dioxide segments for cathode interfacial layer in PLEDs and PSCs

A series of alcohol-soluble amino-functionalized polyfluorene derivatives (PF-N-S, PF-N-SC8 and PF-N-SOC8) comprising various ratios of dibenzothiophene-S,S-dioxide segments (S/SC8/SOC8) in the main chains, respectively, were synthesized and utilized as cathode interfacial layer (CIL) in polymer light-emitting diodes (PLEDs) and polymer solar cells (PSCs) with high-work-function Al (or Au) electrode. The polymers possess LUMO/HOMO levels at −2.78 to −3.53 eV/−5.69 to −6.32 eV. Multilayer PLEDs and PSCs with device configurations of ITO/PEDOT:PSS (40 nm)/P-PPV or PFO-DBT35:PCBM = 1:2 (80 nm)/CIL (3–15 nm)/Al (or Au) (100 nm) were fabricated. The PF-N-S-10/Al (or Au) cathode PLEDs displayed maximum luminous efficiency of 24.4 cd A⁻¹ (or 11.9 cd A⁻¹), significantly higher than bare Al (or Au) cathode device, exceeding well-known Ba/Al and poly[(9,9-bis(3′-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)] (PFN)/Al (or PFN/Au) cathode devices. The enhanced open-circuit voltages (V_ocs), electron reflux and reduced work functions clarify that the electron injection barrier from the Al (or Au) electrode can be lowered by inserting the polymers as CIL. The resulted PSCs also show device performances exceeding Al and PFN/Al cathode devices. The results indicate that PF-N-S, PF-N-SC8 and PF-N-SOC8 are excellent CIL materials for PLEDs and PSCs with high-work-function Al or Au electrode.     

Small molecule interlayer for solution processed phosphorescent organic light emitting device

Using a 4,4′,4′′-tris(N-carbazolyl)-triphenylamine (TCTA) small molecule interlayer, we have fabricated efficient green phosphorescent organic light emitting devices by solution process. Significantly a low driving voltage of 3.0 V to reach a luminance of 1000 cd/m² is reported in this device. The maximum current and power efficiency values of 27.2 cd/A and 17.8 lm/W with TCTA interlayer (thickness 30 nm) and 33.7 cd/A and 19.6 lm/W with 40 nm thick interlayer are demonstrated, respectively. Results reveal a way to fabricate the phosphorescent organic light emitting device using TCTA small molecule interlayer by solution process, promising for efficient and simple manufacturing.     

Matrix influence on the OLED emitter Ir(btp)2(acac) in polymeric host materials – Studies by persistent spectral hole burning

Fundamental photophysical properties of the phosphorescent organometallic complex Ir(btp)₂(acac) doped in the polymeric matrices PVK, PFO, and PVB, respectively, are investigated. PVK and PFO are frequently used as host materials in organic light emitting diodes (OLEDs). By application of the laser spectroscopic techniques of phosphorescence line narrowing and persistent spectral hole burning – improved by a synchronous scan technique – we studied the zero-field splitting (ZFS) of the T₁ state into the substates I, II, and III. Thus, we were able to probe the effects of the local environment of the emitter molecules in the different amorphous matrices. The magnitude of ZFS is determined by the extent of spin–orbit coupling (SOC) of the T₁ state to metal-to-ligand charge transfer (MLCT) states. Only by mixings of MLCT singlets, a short-lived and intense emission of the triplet state to the singlet ground state becomes possible. Thus, sufficiently large ZFS is crucial for favorable luminescence properties of emitter complexes for OLED applications. The analysis of the spectral hole structure resulting from burning provides information about the ZFS values and their statistical (inhomogeneous) distribution in the amorphous matrices. For Ir(btp)₂(acac), we found a significant value of ≈18 cm⁻¹ for the splitting between the substates II and III for all three matrices. Interestingly, for PVK the width of the ZFS distribution is found to be ≈14 cm⁻¹ – almost twice as large as for PFO and PVB. Consequently, for a considerable fraction of Ir(btp)₂(acac) molecules in PVK, the ZFS is relatively small and thus, the effective SOC is weak. Therefore, it is indicated that a part of the emitter molecules shows a limited OLED performance.     

Synthesis and photophysical characterization of an ionic fluorene derivative for blue light-emitting electrochemical cells

A highly fluorescent an ionic fluorene derivative 1 was synthesized and its photophysical, electrochemical and electroluminescence characteristics were investigated. Deep blue emissions were observed for compound 1 in solid as well as in dilute solutions. The synthesized compound shows high fluorescence quantum yield around 77% indicates that compound 1 can perform its role as efficient ionic emitter in LEC devices. Light-emitting electrochemical cell (LEC) devices were fabricated incorporating compound 1 without (device I) and with (device II) ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate (BMIM·PF₆). Devices I and II exhibited blue electroluminescence maximum centered at 455 and 454 nm with CIE coordinates of (0.15, 0.21) and (0.16, 0.22), respectively. Maximum luminance and current efficiency of 1105 cd m⁻² and 0.14 cd A⁻¹ respectively, has achieved for device I while that of device II resulted in 1247 cd m⁻² and 0.14 cd A⁻¹ respectively.     

Ultra-bright alternating current organic electroluminescence

We report on an alternating current (AC) field induced organic electroluminescence (EL) device with internal charge carrier generation and recombination luminance of over 5000 cd m^?2 under AC drive without charge carrier injection from external electrodes. The ultra-bright AC-EL is attributed to an optical optimization performed on the devices via numerical optical simulations based on an optical thin film model as well as an increase in the number of charge carriers achieved via the concept of molecular doping within the device. The luminance levels achieved are highest reported so far in literature for AC organic light emitting devices.